The present invention relates to the field of data communication equipment which interfaces between data terminals and data communication channels. Specifically, the invention relates to data sets including circuits in which transmitted signals are isolated from received signals which are carried over the same data channel and which are of the type employed in full duplex operation.
Data sets (sometimes called modems) which are capable of full duplex operation have been known for a long time. One characteristic of data sets which is of particular interest is their ability to distinguish between transmitted signals and received signals carried over the same data channel.
One type of circuit utilized to isolate transmitted signals and received signals has employed inductive or resistive bridge networks. For telephone lines, the networks make a four-wire to two-wire conversion. Specifically, the two-wire connection to the network is the data channel such as the tip and ring lines in a telephone system. The four-wire connection includes two wires for the transmitted signal and two wires for the received signal.
When transmitted signals and received signals are concurrently present on the data channel, the system is operating in full duplex mode. Bridge networks of the type described have not proved entirely satisfactory in that they do not provide adequate isolation between transmitted and received signals. A problem with such bridge networks results because the resistors or other impedances employed have inherent power losses which degrade the efficiency of the networks. For example, the isolation achievable between transmitted and received signals has been of the order of 12 db. Since in the normal operation of telephone circuits losses of up to 28 db are possible inductive or resistive bridge networks have not proved entirely satisfactory. Further, such bridge networks have been dc coupled to the data link and hence are objectionable in environment where direct coupling is prohibited or otherwise undesirable.
Where dc isolation has been desired, dual transformer circuits have been employed to isolate transmitted and received signals. Again, the dual transformer circuits make a four-wire to two-wire conversion of the type employed in full duplex operation. Dual transformer circuits, however, have not provided adequate separation of transmitted and received signals. As a typical example, isolations using dual transformers have been of the order of 20 db or less which is not adequate in telephone circuits where losses of up to 28 db are normally present.
In order to obtain better isolation between transmitted and received signals, an operational amplifier circuit which achieves a four-wire to two-wire conversion has been employed for full duplex operation. Although, such a network has achieved isolation improvements over prior techniques, still additional isolation between transmitted and received signals is desirable.
Accordingly, it is an objective of the present invention to provide an improved data set which provides greater isolation between transmitted and received signals.